Ethylbenzene is one of the aromatic hydrocarbons that is obtained from naphtha pyrolysis or in reformate. Reformate is an aromatic product given by the catalysed conversion of straight-run hydrocarbons boiling in the 70 to 190° C. range, such as straight-run naphtha. Such hydrocarbons are themselves obtained by fractionation or distillation of crude petroleum oil, their composition varying depending on the source of the crude oil, but generally having a low aromatics content. On conversion to reformate, the aromatics content is considerably increased and the resulting hydrocarbon mixture becomes highly desirable as a source of valuable chemicals intermediates and as a component for gasoline. The principle components are a group of aromatics often referred to as BTX: benzene, toluene, and the xylenes, including ethylbenzene. Other components may be present such as their hydrogenated homologues, e.g. cyclohexane.
Of the BTX group the most valuable components are benzene and the xylenes, and therefore BTX is often subjected to processing to increase the proportion of those two aromatics: hydrodealkylation of toluene to benzene and toluene disproportionation to benzene and xylenes. Within the xylenes, para-xylene is the most useful commodity and xylene isomerisation or transalkylation processes have been developed to increase the proportion of para-xylene.
A further process that the gasoline producer can utilize is the hydrodealkylation of ethylbenzene to benzene.
Generally, the gasoline producer will isolate BTX from the reformate stream, and then subject the BTX stream to xylene isomerisation with the aim of maximising the para-xylene component. Xylene isomerisation is a catalytic process; some catalysts used in this process have the ability not just to isomerise xylenes but also simultaneously to dealkylate the ethylbenzene component. Normally the para-xylene is then separated out to leave benzene, toluene (unless toluene conversion processes have already been applied) and the remaining mixed xylenes, including ethylbenzene. This BTX stream can either be converted by transalkylation to increase the yield of xylenes by contacting with a heavier hydrocarbon stream or can be converted by dealkylation to eliminate selectively ethylbenzene and to increase the yield of benzene, while allowing the xylenes to reach equilibrium concentrations. The latter process is the subject of the present invention.
In ethylbenzene dealkylation at this latter stage of BTX treatment, it is a primary concern to ensure not just a high degree of conversion to benzene but also to avoid xylene loss. Xylenes may typically be lost due to transalkylation, e.g. between benzene and xylene to give toluene, or by addition of hydrogen to form, for example, alkenes or alkanes.
It is therefore the aim of the present invention to provide catalytic materials that will convert ethylbenzene to benzene with a reduced xylene loss.
For the conversion of BTX streams to increase the proportion of closely configured molecules, a wide range of proposals utilizing zeolitic catalysts have been made. One common zeolite group utilized in the dealkylation of ethylbenzene is the MFI zeolites and in particular ZSM-5. The ZSM-5 zeolite is well known and documented in the art.
Many preparation routes have been proposed that provide active MFI zeolites, including ZSM-5, see for example U.S. Pat. No. 3,702,886.
U.S. Pat. No. 4,511,547 proposes a general preparation route for the production of crystalline aluminosilicate zeolites which comprises stirring, whilst heating, an aqueous reaction mixture containing a silica source, an alumina source, an alkali source and an organic carboxylic acid which does not contain an aromatic ring, suitably an organic carboxylic acid having from 1 to 12 carbon atoms. The examples of U.S. Pat. No. 4,511,547 utilise tartaric acid and, from the XRD pattern provided, produce ZSM-5 type zeolite.
Tartaric acid has two chiral centres and exists in four main enantiomeric forms: racemic, meso, levorotatory and dextrorotatory. The racemic form (DL-tartaric acid) is readily available and produced commercially in Europe, South Africa and Japan while the dextrorotatory form (L-tartaric acid) is the commercial product in the USA approved by the FDA for use in the food and pharmaceutical industries (see the Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, Volume 13, pages 1071 to 1078). U.S. Pat. No. 4,511,547 is silent as to the form of tartaric acid utilized but the examples come from Japanese-originating research and it is reasonable to conclude that the racemic form of tartaric acid was used.